1. Field of the Invention
The present invention relates to an optical information reproducing device to reproduce information recorded on an optical information recording medium using interference of a signal beam and a reference beam and a reference beam adjusting method.
2. Description of the Related Art
Hologram recording technology for recording digital information using holography has attracted attention during making a study of next-generation storage technology.
The hologram recording technology is technology for overlapping a signal beam having information of page data modulated two-dimensionally by a spatial light modulator to a reference beam in a recording medium, generating refractive index modulation in the recording medium by a fringe pattern generated at that time, and recording information on the recording medium.
At the time of reproducing information, if the recording medium is irradiated with the reference beam used at the time of recording, a hologram recorded on the recording medium acts like a diffraction grating to generate diffracted light. The diffracted light is reproduced as the same light as the recorded signal beam, including phase information.
The reproduced signal beam is detected two-dimensionally using a light detector such as a CMOS or a CCD. As such, the hologram recording technology enables two-dimensional information to be recorded/reproduced on the optical recording medium at one time by one hologram and enables a plurality of page data to be overwritten to the same place of the recording medium by changing an angle of the reference beam radiated to the recording medium at the time of recording, thereby enabling large-capacity and high-speed information to be recorded/reproduced.
In the reproduction of the hologram, if the recording medium is contracted/expanded according to a temperature, an angle and an interval of a grating recorded as the diffraction grating change, so that a signal quality of reproduction light is deteriorated. In order to compensate for the deterioration in the signal quality, it is necessary to adjust an incidence angle and a wavelength of the reference beam radiated to the recording medium. A method of detecting a characteristic of the reproduction light and adjusting the incidence angle and the wavelength of the reference beam has been suggested (for example, refer to WO 2011/018836 A).